Pipes and tubing of steel can be made by rolling a cylindrical steel blank between the rolls of an inclined-roll mill while piercing the workpiece with an axially stationary piercing mandrel to form a hollow billet. The hollow billet may be further rolled and the result is a seamless pipe or tubing having an internal bore which can be generally of cylindrical configuration and an external peripheral which is likewise generally cylindrical. The process can be that of EP 0 940 193 A2 for example.
The seamless steel billet can be subjected, after piercing, to hot rolling in a succession of rolling mills, the rolling process involving, for example, stretch reduction rolling, in which the diameter of the pipe is reduced and the length thereof increased, reducing rolling and dimensioning rolling. The rolling mills are arranged in a rolling line one after the other and the pipe or tubing thus passes through a succession of mill frames and multiple rolls in each frame engaging the hollow billet.
In each rolling mill stand, there may be a plurality of rolls which contact the hollow billet over respective peripheral segments and in most mill stands of this type of rolling lines, at least three rolls are provided and the result from stand to stand is that the entire periphery of the hollow billet will eventually be contacted by such rolls and a pipe of a reduced diameter determined by the roll and an exact shape, usually an exact cylindrical shape will be obtained.
The pipe produced in this manner should have following the rolling an ideal shape with a cylindrical outer contour and a cylindrical inner contour and such that the inner bore should be as coaxial as possible to the outer periphery. Ideally, therefore, in a section through the pipe, the inner periphery and the outer periphery should form two concentric circles.
Because of the manner in which the pipe is formed, in practice there is always some deviation from perfect concentricity within the permissible fabrication tolerances and as a result a certain permissible eccentricity of the circular contour of the inner periphery relative to that of the outer periphery. The quality parameter in pipe manufacture is thus this eccentricity and since the measure of the eccentricity is a measure of the variation in the pipe wall thickness, during the production process wall thickness measurements are made and monitored. To determine the wall thickness of the pipe, ultrasonic measurement techniques are generally used. The ultrasonic thickness measurement can use a pulse echo method whereby the transit time of an ultrasonic pulse through the thickness of the pipe at a certain location gives the wall thickness.
To determine the eccentricity of a hollow billet, i.e. the product during rolling and before the finished state, i.e. the semifinished product, measurements are required in part to control the process. Indeed, apart from the wall thickness measurement as a quality criterium of the pipe, the eccentricity parameter of the semifinished product is an important further quality indicating parameter.
To obtain a measurement of this parameter as early as possible in the production process, a wall thickness measuring unit can be provided at the outlet of an inclined roll mill. This arrangement allows relatively inexpensively a determination of wall thickness just as the hollow billet emerges from this mill. However, since the hollow billet is rotating at the outlet of the inclined roll mill, a number of wall thickness measurement points can be determined over the periphery of the hollow billet enabling a determination of the eccentricity.
The measured eccentricity of course is the offset of the outer diameter of the hollow billet relative to the inner diameter of its bore. With the standard measurement device as described, however, there is an assumption that this offset is constant over the length of the hollow billet or along the longitudinal coordinate. In practice, however, it is found that the eccentricity varies in the direction of the longitudinal coordinate and indeed has a course which corresponds generally to a helix running along the length of the hollow billet.
This helical course of the eccentricity is a consequence of the rolling of the hollow billet in the inclined roll mill, and the shape of that eccentricity pattern is similar to the shape of a corkscrew. The eccentricity pattern determines a so-called main internal thread whose pitch or twist length is given by the angle of inclination of the rolls of the inclined-roll mill. The eccentricity helix repeats itself periodically at the twist length or pitch. Further eccentricities with greater pitch or lower frequency can be superimposed thereon, for example, as a result of nonuniformity of the heating of the billet in a rotary hearth furnace.
The measurement of the course of the eccentricity over the length coordinate of the hollow billet and thus a determination of the inner surface pattern relative to the outer surface pattern along the length of the hollow billet causes a problem on the following ground:
The main eccentricity, i.e. the main inner thread, whose pitch is given by the advance angle of the inclined roll mill is identical to the measurement spiral. The measurement spiral is understood to be the pattern of the thickness measuring point with which the thickness measurement device scans the hollow billet. Since normally the thickness measuring device is located at a fixed point along the path of the hollow billet and mandrels, the wall thickness of the point of the hollow billet juxtaposed with the thickness-measuring device, because the hollow billet upon emergence from the inclined roll mill both rotates and moves longitudinally past the measurement point, the measurement point describes a spiral in space along the hollow billet. The latter is the measurement spiral along which the wall thickness measurements are taken.
Because of the identity of the loop length of the main internal thread with the pitch of the measurement spiral, it is not possible to obtain, with such a wall thickness measurement device, a determination of the spiral pattern of the eccentricity along the length coordinate itself. An important aspect of the eccentricity of the hollow billet cannot be determined and it is not possible to draw conclusions as to the spatial distribution of the eccentricity in regions other than the region of the measurement spiral. Such information is required for a true evaluation of the quality of the product and in many cases for adequate control of the rolling process.